Thermal recovery of viscous oil with selectively spaced fractures



May 28, 1968 n.. K. STRANGE ET AL THERMAL RECOVERY OF VISCOUS OIL WITHSELECTIVELY SPACED FRACTURES Filed OCT.. 25, 1966 2 2l |6 l8\f F tifINVENTORS LLOYD K. STRANGE DQYLE G. MARRS Bv?A )@MLM ATTCRNEY UnitedStates Patent O 3,385,362 THERMAL RECOVERY F VISCOUS OIL WITHSELECTIVELY SPACED FRACTURES Lloyd K. Strange, Grand Prairie, and DoyleG. Marrs,

Corpus Christi, Tex., assignors to Mobil Oil Corporation, a corporationof New York Filed Oct. 26, 1966, Ser. No. 589,615 4 Claims. (Cl. 166-11)This invention relates to recovering hydrocarbons from a subterraneanformation. More particularly, it relates to an in situ combustion methodfor hydrocarbon recovery.

Many a subterranean formation contains viscous hydrocarbons not readilyproduced by primary recovery methods. These methods require hydrocarbonsmovable through the formation by natural reservoir forces in suflicientamounts to be produced commercially from wells by pumping or other fluidlifting means. With secondary recovery methods, external energy issupplied t0 the fluids in a formation to facilitate their movement toproduction wells. One useful secondary recovery method employs the insitu combustion of in place combustibles, to provide heat and toincrease fluid pressures, as the external energy needed to flow viscoushydrocarbons through the formation into production wells.

There are problems with using in situ combustion in a secondary recoverymethod fo-r a thick formation which contains viscous hydrocarbons thatdo not flow under natural formation forces. Under certain conditions, adirect combustion front can suffer from a liquid block which effectivelyshuts off the cocurrent flow of combustion-'supporting gases required tomaintain such front. Inverse, or reverse, combustion fronts can beemployed to avoid some liquid blocking difllculties. However, excessiveamounts of in place hydrocarbons are consumed by the reverse combustionfront. Also, production wells are subjected to destructive temperaturesand corrosive 'gases which begin to reduce seriously their useful livesfrom the moment such reverse front originates. The direct and reversecombustion fronts may be combined into an in situ combustion recoverymethod. However, known methods using such combined fronts have notprovided what we consider to be a satisfactory mode of operation andresult.

It is therefore an object of this invention to provide a method forrecovering viscous hydrocarbons from a thick formation which overcomesthe above-stated problems and difliculties. Another object of thismethod is subterranean heating of a thick formation by successivecounterdirectionally moving combustion fronts whose movement is arrangedto be cooperative with gravity drainage of in place fluids to providefor the effective recovery of visous hydrocarbons. Another object is tocontrol iluid flows in this method by regulating the movement of directand reverse combustion fronts to avoid liquid blocks, enhance gravitydrainage and simplify well operations without sacrificing the improvedrecovery of viscous hydrocarbons. These and other objects will beapparent when considered in conjunction with the following detaileddescription of a preferred embodiment of the method of this invention,the claims, and the appended drawings, wherein:

FIGURE 1 is a vertical section of a subterranean formation containingviscous hydrocarbons after practice of initial steps of this invention;and

FIGURE 2 is an illustration of the arrangement of FIGURE 1 during thepractice of latter steps of this invention.

In accordance with this invention, there is provided a method forrecovering viscous hydrocarbons from a thick formation. Initial stepsare practiced through spaced- 3,385,362 Patented May 28, 1968 ICC apartinjection and production wells penetrating into the formation whereby aheated zone is produced, by reverse combustion, extending from theproduction well toward the injection well in the upper portion of theformation. As as result, heat is concentrated about the production well.Thereafter, a direct combustion front is established vertically throughthe formation adjacent the injection well. This front is moved towardthe production Well. Selectively placed fractures are used with thereverse and direct combustion fronts in a cooperative manner so thatgravity drainage, and thermally stimulated, flows of the viscoushydrocarbons are directed in a substantially uniform manner toward theproduction well which accepts fluid flows from the lower part of theformation. Hydrocarbons are recovered with this flow of iluid from theproduction well.

In the drawings and the following description, corresponding structureswill be designated with the same nomenclature and numeral.

Referring to FIGURE 1 of the drawings, a description of the preferredembodiment of the met-hod of this invention will be given in referenceto a subterranean formation 11 which contains viscous hydrocarbons thatcannot be readily produced by primary recovery methods. The formation 11resides beneath the earths surface 12 between an overburden strata 13and supporting strata 14. For purposes of this invention, the strata 13and 14 may be considered to be impervious to fluid flows `and barren ofhydrocarbons.

The formation 11 preferably has no significant horizontal barriers whichlimit vertical fluid flows. Particularly, the formation 11 should befree of shale, silt stone, and other types of horizontally disposedbarriers, which disrupt vertical fluid flows. Also, the formation 11should be of a relatively great thickness, and preferably the thicknessis 40 feet or greater. In any event, the for-mation 11 should havesuflicient thickness that the effects of gravity flow on heat-thinnedhydrocarbons are significant when compared to hydrocarbon flows over thedistance between the 'wells employed in this method for producing thesehydrocarbons. The formation 11 is exemplified in its -desiredcharacteristics vfor practicing this method by the oil sands of theAthabasca River deposit, and the heavy oil sands of California whereinin situ combustion is presently being employed as a means for theproduction of petroleum.

As a first step of the present method, well means are provided forconveying fluids between the earths surface 12 and into the formation11. The well means may be provided by injection and production wells 16and 17, respectively, penetrating the formation 11. The wells 16 and 17are spaced apart at least the thickness of the formation 11. Theinjection well 16 extends through the entire thickness of the formation11 to terminate in a fluid-seal on the strata 14. The well 16 includes acasing 18 cemented by a cement bond 19 to the adjacent strata 13 andformation 11. The casing 18 carries a Wellhead assembly 21 through whichpasses a conduit 22 for conveying fluids between the earths surface 12and the interior of the well 16. The production Well 17 is arrangedwithin the formation 11 so that fluid flows may be taken initially onlyfrom the upper half of the formation 11. As illustrated in FIGURE 1, thewell 17 penetrates only the upper half of the formation 11 duringpractice of the initial steps of this method. The well 17 has a casing23 extending into the formation 11. The casing 23 obtains fluidcommunication with the formation 11 via a plurality of perforations 24.The casing 23 is sealed by a cement bond 26 to the strata 13. A plug 27seals the bottom of the casing 23 to fluid flows. Wellhead assembly 28resides at the upper end of the casing 23. A conduit 29 for conveyingfluids between the earths surface 12 and the formation 11 extendsthrough the wellhead assembly 28.

As the next step, a fluid-entry is provided from the well 16 into theupper half of the formation 11.` The fluidentry may be a horizontalfracture extending from an opening in the casing 18 and cement bond 19.For this purpose, the casing 18 and cement bond 19 are notchedcircumferentially in the horizontal by suitable procedures, for example,as is shown in U.S. Patent 3,050,122. After notching the well 16, ahorizontal fracture 33 is extended therefrom at the level of the notchsubstantially across the formation 11 to a position spaced from theproduction well 17. Any means for providing the horizontal fracture 33may be employed. For example, a fracturing fluid can be pressurizedthrough the conduit 22 into the well 16 until the resultant fluidpressures cause the formation 11 to fracture horizontally at the levelwherein the casing 18 and cement bond 19 are notched. Other means ofproviding the fracture 33 may be used if desired. In many cases, thefracture 33 will serve as a fluid-entry extending from the injectionwell 16 without being held open. However, better results can be obtainedby filling the fracture 33 with a propping material 34 to facilitatesustained fluid flows therethrough.

The fracture 33 is spaced at its closest extremity a suflicient distancefrom the production well 17 that a reverse combustion front createdadjacent thereto can expand through the vertical extent of the upperhalf of formation 11 by the time this front has reached the fracture 33.In practical operations, the fracture 33 will extend from the well 16about 50-75 percent of the distance to the production well 17. Thisrange of horizontal extensions of the fracture 33, in most instances,obtains the aforementioned vertical expansion of the reverse combustionfront moving from the production well 17 toward the injection well 16.For maximum heating and gravity flow of fluids, the fracture 33 ispreferably placed at a level spaced a distance one-fourth the thicknessof the formation 11 from its upper boundary.

As the next step, a combustion front is established vertically acrossthe upper half of the formation 11 adjacent the production well 17. Anysuitable means for obtaining such front may be used. For example, acombustionsupporting gas is introduced, under sufficient pressure, intothe conduit 22 of the injection well 16 to flow through the fracture 33,the intervening formation 11, and through the perforations 24 into theproduction well 17. The formation 11 adjacent the perforations 24 at thewell 17 is then ignited by a heating means to produce the reversecombustion front.

Alternatively, after fluid `communication is established between thewells 16 and 17, combustion-supporting gas may be flowed temporarilythrough the conduit 29 of the production well 17. Then the formation 11adjacent the perforations 24 is ignited by suitable means to form adirect combustion front across the vertical extent of the upper half ofthe formation 11. Immediately thereafter, the combustion-supporting gasis passed solely from the injection well 16 to the production well 17 toconvert the direct combustion front into a reverse combustion front withthe combustion products being produced from conduit 29 in the productionwell 17.

The reverse combustion front is moved, by continued passage of thecombustion-supporting gas, from the production well 17 toward theinjection well 16 until the front extends at least the vertical extentof the upper half of the formation 11 and also extends horizontally toadjacent the closest extremity of the fracture 33. However, the frontmay be moved until it extends substantially across the formation 11between the wells 16 and 17 with a corresponding increased consumptionof hydrocarbons.

During the time the reverse combustion front is moving from theproduction well 17, fluids are removed through this well solely from theupper half of the formation 11, as was mentioned earlier. The foregoingsteps provide for the production of a heated zone 36 (denoted byshading) in the formation 11 adjacent the well 17 in which thehydrocarbons are heated sufficiently that no liquid blocks occur andgravity drainage is enhanced.

The production well 17, for carrying out the foregoing steps, may be ofsimple and inexpensive design. This construction is of advantage sincethe well 17 is subject to severe corrosive and thermal attack from theheated fluids produced by the reverse combustion front. In manyinstances, the lower perforated extremity of the casing 23 in the well17 will be destroyed in carrying out the reverse combustion front whichprovides the heated zone 36 but only after it has served its neededpurpose. The simple design for the well 17 is further facilitated by theproduction of a mixture of liquids and gases in the conduit 29. Thiscombined fluid stream allows the use of a single conductor for removingfluids from the well 17.

The concentration of heat in the heated zone 36, especially closelyadjacent the production well 17 is of significant advantage. The heatconducted, from the heated zone 36, to the lower portion of theformation 11 permits more ready fluid flows therethrough. In theremaining steps hereinafter described, the hydrocarbons in the heatedzone 36 readily flow by gravity to the lower extremities of theformation 11. Thus, the viscous hydrocarbons, in the formation 11adjacent the production well 17, are placed into a condition of flowingmore easily than those more closely adjacent the injection well 16. Thiscondition results in a more uniform sweeping of the formation 11, by thesubsequent steps of this invention, for producing a large portion of thehydrocarbons in the formation 11 between these wells.

The well 17 is now arranged, as illustrated in FIGURE 2, for fluidcommunication solely with the lower half of the formation 11. Anysuitable structures may be used for this result. For example, aninterliner 38 is disposed within, and fluidly sealed, at its upperextremity to the casing 23. Usually, various types of hangers are usedfor this construction which are well known to those skilled in the artbut not shown in the present drawings, or described in the description,of this method. The interliner 38 is sealed against fluid leakage on topof the strata 14. The cement bond 26 is then extended the full depth ofthe formation 11. Similarly, the conduit 29 is extended into the lowerextremities of the production well 17.

After the well 17 has been arranged to accept fluids only from the lowerportion of the formation 11, a fluidentry is provided into the lowerhalf of the formation 11. For this purpose, a circular notch in thehorizontal is formed in the well 17 at its lower extremity. Preferably,the notch is at, or closely adjacent, the lower bedding plane of theformation 11. Thereafter, by suitable means, a horizontal fracture 39 isextended at the level of the notch through the formation 11 toward theproduction well 17. The same, or different, means may be employed forproducing the fracture 39 as were employed in providing the fracture 33.The fracture 39 extends horizontally in the formation 11, at the desiredlevel, so as to reside not closer at any location in the formation tothe fracture 33 than the distance d which is less than aboutthree-fourths the thickness of the formation 11. Furthermore, thefracture 39 should extend from the production well 17 toward theinjection well 16 so as to reside not closer at any location to anyfluid-entry into the formation 11 from the injection well 16 a distanced which is wherein kv=kh, and

dem@

wherein Kh Kv In the equations, KV is the permeability to fluids in thevertical and Kh is the permeability to fluids in the horizontal in theformation 11. By this means of spacing the fracture 39 from the fracture33, and also any other fluid-entry from the well 16, the problem ofpremature breakthrough of fluid flows effected from the well 16 into thefracture 39 of the production well 17 will be avoided.

As the next step, a combustion front extending vertically through theformation 11 along the injection well 16 is provided by any desiredmechanisms. For this purpose, it may be desired to provide at least oneadditional iiuid-entry from the injection well 16 into the formation 11below the level of the fracture 33. The Huid-entry may be perforationsthrough the casing 18 and cement bond 19 similar to the perforations 24of the production well 17. However, it is preferred that the fluid-entrybe provided by at least one, or more, additional horizontal fractures 41and 42. The fractures 41 and 42 may be provided by any suitable means.The procedure described for producing the fracture 33 may be also usedfor producing the fractures 41 and 42. It is to be noted that thecriterion of spacing the fracture 39, and any other uid-entry from thewell 16, from the fracture 33 is likewise applicable to the fractures 41and 42. Suitable means are used for initiation of the combustion frontin the formation 11 along the injection well 16. Such means may beprovided by circulating the combustion-supporting gas into the well 16via conduit 22 so that the front is supplied by auto-ignition. Usually,some heating means within the injection well 16 will be used for heatingthe adjacent parts of the formation 11 to ignition temperatures. Bythese means, a combustion front is provided which extends substantiallythe vertical thickness of the formation 11 adjacent the injection well16.

Thereafter, the ow of the combustion-supporting gas is continued intothe well 16 and flows through the formation 11 toward the fracture 39 ofthe production well 17. The combustion front moves cocurrently with theflow of the combustion-supporting gas to produce a heated zone 44. Theheated zone 44, denoted by shading in the formation 11, resides attemperatures wherein liquid blocks are avoided and gravity drainage isaccelerated. The heated zone 44 is bounded forwardly by an incline whichadvances at uniform rates vertically and horizontally for movin-g theviscous hydrocarbons from the form-ation 11 into the fracture 39. Thisarrangement prevents a premature breakthrough of the injectedcombustion-supporting gas from any uid-entry such as one of thefractures 33, 41, and 42, into the fracture 39. As a result, nosubstantial amounts of hydrocarbons in the form-ation 11 are bypassed.Similarly, the combustion-supporting gas flows principally toward thefracture' 39. This is of advantage in that the displaced viscoushydrocarbons drain into the fracture 39, and the concurrent gas flowlowers the liquid saturation in the formation 1'1 adjacent the fracture39. This functioning prevents liquid blocks from occurring in unheatedzones in the formation 11 during the movement of hydrocarbons before thedirect combustion front toward the production well 17. Additionally, thefluids produced from the fracture 39 into the well 17 contain bothheated gaseous combustion products and liquid hydrocarbons. Thiscombined ti-ow reduces the temperature of the hot gases in theproduction well 17 to protect it substantially against destruction.Additionally, the pantieular spacings between the horizontal fractures,or any other fluid-entry, extending between the wells 16 and 17, avoidthe direct interc-ommunications between these fractures. This resultprovides for the maximum vertical and horizontal coverage of the directcombustion front in its movement away from the injection well 16 beforeits breakthrough into the production well 17 Another step is therecovering of fluids containing hydrocarbons from the production well 17solely from the lower half of the formation 11. These iiuids lareconveyed through the conduit 29 to the earths surface for ultimateutilization. The arrangement of the production well 17 is of especialutility -for producing fluids from the formation 11 since simplesrtuctures are usable. Fluid ows are easily controlled in the well 17without sophisticated apparatus for segregating extremely hot combustiongases from liquid hydrocarbons. If it is desired, a liquid level may bemaintained in the production well 17 adjacent its lower extremity as aseal. The seal provides for immediate displacement of produced fluidsinto the conduit 29. This also provides a safer means of operation byavoiding a build-up of explosive gaseous mixtures within the productionwell 17 It is intended that the foregoing description be taken inillustration, and not in limitation, of this invention. Various changesand alterations in the described steps may be made without departingfrom the spirit of this invention. It is intended that such arrangementsbe considered within the scope of the appended claims; and that theseclaims define the method of this invention.

What is claimed is:

1. A method for recovering viscous hydrocarbons from a thicksubterranean formation without significant horizontally disposedbarriers which limit vertical fluid iiows, the steps comprising:

(a) providing spaced-apart injection an-d production well meanspenetrating into the hydrocarbon-bearing formation for conveying fluidsbetween the earths surface and said formation;

(b) providing in the upper half of said formation a horizontal fractureas a fluid-entry extending from said injection well means substantiallyacross said formation to a position spaced from said production wellmeans a distance in which a reverse combustion front can expandvertically across the upper half of the formation;

(c) establishing a combustion front vertically across the upper half ofsaid formation adjacent said production well means, and then moving saidfront toward said injection well means by passing acombustion-supporting gas from said injection to said production wellmeans until said front extends at least the vertical extent of saidupper half of said formation and horizontally to adjacent the terminusof said fracture;

(d) removing fluids from said production well means solely at the upperhalf of said formation;

(e) providing a horizontal fracture extending from said production wellmeans toward said injection well means in the lower half of saidformation and adjacent its lower bedding plane, said fracture extendinghorizontally in said formation from said production well means to residenot closer at any location to the first-mentioned fracture extendingfro-m said injection well means a distance d less than aboutthree-fourths the thickness of said formation and said horizontalfracture extending from said production well means yresiding not closerat any location to any Huid-entry location into said formation from saidinjection well means a distance a" defined by equations (l) and (2)wherein Kh Kv Where in the equations Kv is the permeability to fluids inthe vertical and Kh is the permeability to fluids in the horizontal insaid formation;

.(f) establishing a combustion front extending vertically in saidformation yalong said injection well means and moving said front towardsaid production well means with a concurrent iiow ofcombustion-supporting gas, and

(g) recovering fluids containing hydrocarbons from said production wellmeans solely at the lower half of said formation.

2. The method of claim 1 wherein said fracture obtained in step (b)extends from about 50 to 75 percent of the distance between said wellmeans.

3. The method of claim 1 wherein prior to establishing said combustionfront in step (f), at least one additional horizontal fracture isprovided -as a duid-entry extending from said injection well meanstoward said production well means and at a lower level than saidhorizontal fracture obtained in step (b).

4. The method of claim 1 wherein said horizontal fracture obtained instep (b) resides at a level spaced from the upper boundary of saidformation one-fourth the thickness thereof, and said horizontal fractureobtained in step (e) resides at a level spaced from the lower boundaryof said formation not more than one-fourth the thickness thereof.

References Cited UNITED STATES PATENTS Tadema 166-11 Campion et al.166-11 Prentiss 166-11 X Tek et al 166-11 Terwillnger et al 166-11 Reedet al 166-11 Kuhn 166-11 Reed et al. 166-11 Sha-rp 166-11 Prats 166-11 XStrange et al 166-11 15 STEPHEN J. NovosAD, Primary Examiner.

1. A METHOD FOR RECOVERING VISCOUS HYDROCARBONS FROM A THICKSUBTERRANEAN FORMATION WITHOUT SIGNIFICANT HORIZONTALLY DISPOSEDBARRIERS WHICH LIMIT VERTICAL FLUID FLOWS, THE STEPS COMPRISING: (A)PROVIDING SPACED-APART INJECTION AND PRODUCTION WELL MEANS PENETRATINGINTO THE HYDROCARBON-BEARING FORMATION FOR CONVEYING FLUIDS BETWEEN THEEARTH''S SURFACE AND SAID FORMATION; (B) PROVIDING IN THE UPPER HALF OFSAID FORMATION A HORIZONTAL FRACTURE AS A FLUID-ENTRY EXTENDING FROMSAID INJECTION WELL MEANS SUBSTANTIALLY ACROSS SAID FORMATION TO APOSITION SPACED FROM SAID PRODUCTION WELL MEANS A DISTANCE IN WHICH AREVERSE COMBUSTION FRONT CAN EXPAND VERTICALLY ACROSS THE UPPER HALF OFTHE FORMATION; (C) ESTABLISHING A COMBUSTION FRONT VERTICALLY ACROSS THEUPPER HALF OF SAID FORMATION ADJACENT SAID PRODUCTION WELL MEANS, ANDTHEN MOVING SAID FRONT TOWARD SAID INJECTION WELL MEANS BY PASSING ACOMBUSTION-SUPPORTING GAS FROM SAID INJECTION TO SAID PRODUCTION WELLMEANS UNTIL SAID FRONT EXTENDS AT LEAST THE VERTICAL EXTENT OF SAIDUPPER HALF OF SAID FORMATION AND HORIZONTALLY TO ADJACENT THE TERMINUSOF SAID FRACTURE; (D) REMOVING FLUIDS FROM SAID PRODUCTION WELL MEANSSOLELY AT THE UPPER HALF OF SAID FORMATION; (E) PROVIDING A HORIZONTALFRACTURE EXTENDING FROM SAID PRODUCTION WELL MEANS TOWARD SAID INJECTIONWELL MEANS IN THE LOWER HALF OF SAID FORMATION AND ADJACENT ITS LOWERBEDDING PLANE, SAID FRACTURE EXTENDING HORIZONTALLY IN SAID FORMATIONFROM SAID PRODUCTION WELL MEANS TO RESIDE NOT CLOSER AT ANY LOCATION TOTHE FIRST-MENTIONED FRACTURE EXTENDING FROM SAID INJECTION WELL MEANS ADISTANCE D LESS THAN ABOUT THREE-FOURTHS THE THICKNESS OF SAID FORMATIONAND SAID HORIZONTAL FRACTURE EXTENDING FROM SAID PRODUCTION WELL MEANSRESIDING NOT CLOSER AT ANY LOCATION TO ANY FLUID-ENTRY LOCATION INTOSAID FORMATION FROM SAID INJECTION WELL MEANS A DISTANCE D'' DEFINED BYEQUATIONS (1) AND (2):